Electromechanical Car horn

Thread Starter

SantyEE

Joined Mar 16, 2015
3
Hey All! I am a new joinee at a car horn manufacturing factory, I needed help in understanding the L,C,R relationship for the coil in
the 12V DC horn 1.5A max.The make break frequency for the contacts is between 350-450/second.

To help better understand the working attached the 3d image
Thanks!
HORN.jpg
 

kubeek

Joined Sep 20, 2005
5,744
I doubt there is any C in the horn, and the L of the magnet coil is not really part of the equation. It works the same way as a doorbell, and the frequency is dictated by the wight and springiness of the diapraghm and the distance between the contacts.
 

#12

Joined Nov 30, 2010
18,217
This could be a fun physics problem where you figure out how inertia and spring stiffness resemble L, R, and C components. The electrical L, R, and C, are mostly irrelevant to the operation. For instance, the inductance of the coil is long past its time constant when the diaphragm moves the contacts and there is no physical capacitor in there at all.

ps, I have used these for a 4 story door bell/phone ring system, and they suck for long term use. Half the contacts were destroyed within 5 years.:(
 

Thread Starter

SantyEE

Joined Mar 16, 2015
3
Thanks for the reply guys.

Some models use a capacitor to reduce the arcing and thereby increase life of the contacts, some have a diode across the contacts for EMI suprression requirements.

I had read about the L/R ratio and thought it played a important role in DC coil design.

Any clue about the inrush spike, when supply is turned on and the increase in overall resistance of the horn
 

Hypatia's Protege

Joined Mar 1, 2015
3,226
Any clue about the inrush spike, when supply is turned on
There will be no 'inrush surge' in the 'traditional' sense of the term -- howbeit the mean current will 'settle' to less than E[applied]/R[coil] owing to the sub-unity duty cycle corollary to periodic interruption of the circuit (at the AF rate of the buzzer)...

and the increase in overall resistance of the horn
Coil heating (with subsequent resistance rise) will be negligible in a properly designed EM buzzer operated as per specification....

Best regards
HP
 

Thread Starter

SantyEE

Joined Mar 16, 2015
3
Yes Aleph(0), every time the horn is switched on there is a surge apart from the EMF generated, I plan to analyse this on an oscilliscope.
But need general pointers to good design to keep the surge as well as emf low without using the capacitors or diodes to reduce manufacturing costs.

HP, these horn are designed to work for 100,000 cycles (1second one and 2 second off) without need for adjusting the contacts
 

GopherT

Joined Nov 23, 2012
8,012
Yes Aleph(0), every time the horn is switched on there is a surge apart from the EMF generated, I plan to analyse this on an oscilliscope.
But need general pointers to good design to keep the surge as well as emf low without using the capacitors or diodes to reduce manufacturing costs.

HP, these horn are designed to work for 100,000 cycles (1second one and 2 second off) without need for adjusting the contacts
You can measure the DC resistance of the coil,
you can measure the capacitance of the horn.
From those valuves, propose a circuit and mH rating of the inductor that is needed to resonate at 350 to 400 Hz.

Then find an inductor of similar ratings on Digikey.com, Newark.com or other electronics distributors. Does it look like your horn? How big is it? Do you think the horn resonates at 350 to 400Hz because of mechanical or electrical effects?
 
HP, these horn are designed to work for 100,000 cycles (1second one and 2 second off) without need for adjusting the contacts
It seems you misunderstood my reply (my bad:oops:) -- To wit, upon application of power, current is equal to EMF[PSU]/Resistance[coil] which being 'mitigated' by instantaneous (inductive) reaction followed by attainment of a (lower) 'mean' as per the switching duty cycle (i.e. the ratio of 'contacts closed' to 'contacts open' times) attending the mechanical oscillation of the diaphragm.

But need general pointers to good design to keep the surge as well as emf low without using the capacitors or diodes to reduce manufacturing costs.
Given your conditions, increased coil resistance is about your only means to that end (not likely an economically competitive solution) - In any case, effective transient suppression is essential is damage to semiconductor components (elsewhere in the electrical system) to be avoided!

Best regards and good luck
HP:)
 
You can measure the DC resistance of the coil,
you can measure the capacitance of the horn.
From those valuves, propose a circuit and mH rating of the inductor that is needed to resonate at 350 to 400 Hz.
Aye! But be advised -- It'll require a BIG coil inductance to 'tune' the distributed/incidental capacitance to Ca. 4ooHz! :eek::D

Best regards
HP:)
 

Aleph(0)

Joined Mar 14, 2015
597
Aye! But be advised -- It'll require a BIG coil inductance to 'tune' the distributed/incidental capacitance to Ca. 4ooHz! :eek::D

Best regards
HP:)
HP I agree! cuz it would take like 160 HENRIES of inductance to resonate with 1nf (which I say is liberal estimate) of stray capacitance at 400hz:eek:! And then anyhow the Q would be so low it wouldn't matter:(
 
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